The present invention relates generally to a method for determining the chemical composition of fine features, such as surface particles on semiconductor wafers. During the manufacturing test of semiconductor wafers, the location of small defects may be detected using a variety of methods. Frequently, the composition of the defect yields important information about the manufacturing process. Such information may assist, for example, in isolating problems in the manufacturing process or discovering contamination from errant debris in the manufacturing environment.
Others have proposed determining the composition of small defects using xe2x80x9cenergy-dispersive spectroscopyxe2x80x9d, or EDS, as described in relation to FIG. 1. In an EDS system 10, an incident electron beam 20 is directed towards a known defect location 22 at a fixed energy, generally 5-20 keV. X-rays 24 are emitted from the defect particle and detected by an x-ray detector 30. Chemical identification of a defect is made by resolving the energy of x-rays emitted from the sample as a result of the irradiation.
The incident beam 20 in EDS system 10 has a fixed energy and current, and all of the x-rays emitted by the sample are detected simultaneously by detector 30 and their energies are resolved. X-ray emissions occur only when the incident electron energy exceeds a minimum threshold energy level sufficient to knock electrons out of particular levels. FIG. 2 shows a diagram plotting the intensity of the x-rays detected by detector 30 across x-ray energies for a sample including aluminum (Al), silicon (Si), copper (Cu) and tungsten (W), all commonly used in the manufacture of semiconductor devices. Around the x-ray energy level of Al a peak intensity 50 indicates an increased level of x-rays emitted from the sample location. Similarly a peak intensity 54 is indicated around the energy level of Cu.
A smaller intensity peak 52 is formed around the area of energy level of Si. The difference between the x-ray energy level of Al and Si is only about 250 eV. Because the resolving power of the detectors used in EDS is never better than 50 eV, and is frequently worse than 150 eV, a great deal of data must be collected in order to resolve two peaks so close together.
EDS systems also present another problem. For most atomic species, the 5-20 keV supplied by the incident beam 20 of an EDS system is much greater than that necessary to excite the electrons, so the depth of material probed is usually 0.5 to 5 microns. However, a small defect particle may be as little as 0.1 microns deep. For a small defect particle 22, as shown in FIG. 1, the beam may therefore probe the substrate 12 as well as the defect 22. As beam 20 penetrates the substrate 12, the electrons of the beam are deflected throughout a volume 26, that may be 1-10 microns wide, exciting other electrons in the volume. X-rays 28 are emitted from atoms throughout the probed volume. It is impossible to discern between the x-rays emitted from the particle or from the substrate, so it cannot be determined if any chemicals detected are from the errant particle, or are from the substrate. Some have solved this problem by testing at the particle location, and testing a second time near the particle area to resolve the difference, but this process requires additional processing time and may introduce other errors if the neighboring area tested has different structures from the defect area.
Another technique that has been used by others to determine the chemical composition of defects is Auger Electron Spectroscopy (AES), illustrated by system 70, shown in FIG. 3. In an AES process, a fixed electron beam 72 of 3-5 keV is projected towards particle 22 on substrate 12. Auger electrons 74 are emitted from particle 22 and detected by an electron energy analyzer 76. The energy of Auger electrons is well known, and is fixed for each atomic species.
Although the beam 72 may penetrate the particle 22 and substrate 12, Auger electrons, unlike x-rays, are only released from the surface area struck by the beam. An AES system effectively probes only about 0.005-0.05 microns into the particle. Although this solves the problem in EDS of probing the substrate below the defect particle, it raises additional implementation problems. Since such a shallow amount of the surface is probed, it is very important that the surface is not contaminated with even a minute residue of other material. For example, condensed water in the air will affect the chemical determination. This problem has been reduced by cleaning the surface by ion-bombardment 82, and holding the material in an ultra-high vacuum chamber 80 at 10xe2x88x929 Torr or lower before measurements are made.
The specialized equipment needed for AES systemsxe2x80x94ultra high vacuum system and electron energy analyzerxe2x80x94is very expensive, and requires highly-trained operators. Auger systems, because of the need for ultra-high vacuum control, also cannot be easily retrofit to existing manufacturing equipment. AES is therefore not suitable for in-line analysis of semiconductor product wafers, and is instead commonly used for failure analysis of semiconductor devices.
A third method employs variations of electron appearance spectroscopy to the problem of identifying materials. For example, Park and Houston, in xe2x80x9cAPPEARANCE POTENTIAL SPECTROSCOPY ON AN AUSTERE BUDGETxe2x80x9d, SURFACE SCIENCE, 26 (1971) (pages 664-666), Letters to the Editor, describe a simple appearance spectroscopy system in which the derivative of the photocurrent as a function of the sample potential exhibits sharp peaks at the potentials corresponding to the threshold energies. Chopra and Chourasia in xe2x80x9cAPPEARANCE POTENTIAL SPECTROSCOPY OF SOLID SURFACESxe2x80x9d SCANNING MICROSCOPY, Vol. 2, No. 2, 1988 (pages 677-702), review appearance spectroscopy and survey some applications.
These and other works in appearance spectroscopy are based upon a simple spectrometer employing a tungsten filament which provides electrons which impinge on the sample to be studied. A grid electrically separates the filament and the detector assembly, and x-rays passing through the grid strike the walls of the chamber. The resulting photoelectrons are collected on a positive electrode. The resulting signal is amplified and synchronously detected by a phase-lock amplifier.
Previous work in appearance spectroscopy, however, has failed to consider a focused electron beam projected towards particular locations on the specimen, but instead found the properties of materials over large areas.
In accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method of and an apparatus for determining the chemical composition of a feature on a substrate, comprising the steps of directing a focused electron beam towards the feature, thereby causing the feature to emit x-rays, detecting x-rays emitted from the feature, while varying the energy of the beam and maintaining the focus of the beam on the feature, and determining the composition of the feature. The electron beam may be scanned over a surface of the substrate or stepped over the surface of the substrate to locations corresponding substantially to predetermined defect sites on the substrate.
The method may additionally comprise sequentially directing the focused electron beam towards each of a plurality of features on the substrate, sequentially detecting x-rays emitted from each of the features while varying the energy of the beam, and determining the composition of each of the features. The composition of the feature may be determined by monitoring the relative intensity of the x-rays while varying the energy of the beam. The energy may be gradually increased while the x-rays are detected. The composition of the feature can be determined from a lock-in signal corresponding to a derivative of the intensity of the x-rays. The use of lock-in detection as described herein includes, without limitation, those methods described on pages 460-461 of xe2x80x9cBuilding Scientific Apparatusxe2x80x9d (Second Edition) by Moore el al., published in 1989 by Addison Wesley, the description of which is incorporated herein by reference. Other averaged forms of detection may also be used.
The beam may be sequentially varied through predetermined energy levels selected to produce an increase in x-ray emission intensity from known materials. The substrate may be maintained at a substantially constant voltage, preferably of about zero volts, for example by a gas jet directed at the substrate. Alternatively, an electron beam, preferably a de-focused electron beam having a very low landing energy, may be used to maintain the substrate at a substantially constant voltage.
Another aspect of this invention is a method for determining the chemical composition of a feature, comprising directing a focused electron beam towards the feature, the electron beam having a predetermined energy corresponding to a value which causes an element to emit electrons, detecting electrons emitted from the feature, while varying the energy of the beam in a range around the predetermined energy and while maintaining the focus of the beam on the feature, and determining the composition of the feature.
A further aspect of this invention is an apparatus for determining the chemical composition of a feature on a substrate, comprising: a means for directing a focused electron beam towards the feature, thereby causing the feature to emit x-rays; a means for detecting the x-rays emitted from the feature, while varying the energy of the beam and maintaining the focus of the beam on the feature; and a means for determining the composition of the feature. The means for directing the focused electron beam may include an electron gun or other system for producing electrons, and focusing electronics, which may include software or hardware which is used to control the focus of the beam while varying its energy. The means for detecting x-rays may include any suitable x-ray detector, such as a scintillator or any other solid state semiconductor detector. The means for determining the composition of the feature may include software or hardware, such as a programmed processor (with or without additional memory) or a hardwired system, to analyze the data from the detector.
Still another aspect of this invention is an apparatus for determining the chemical composition of a feature on a substrate, comprising: a means for directing a focused electron beam towards the feature, thereby causing the feature to emit electrons; a means for detecting the electrons emitted from the feature, while varying the energy of the beam and maintaining the focus of the beam on the feature; and a means for determining the composition of the feature. The means for directing the focused electron beam may include an electron gun or other system for producing electrons, and focusing electronics, which may include software or hardware which is used to control the focus of the beam while varying its energy. The means for detecting emitted electrons may include a channeltron, an anode, or a system which measures current at the sample. The means for determining the composition of the feature may include software or hardware, such as a programmed processor (with or without additional memory) or a hardwired system, to analyze the data from the detector.
Objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.